An example of a thermally insulated double pipe used in a superconducting electric power transmission is illustrated in FIG. 1. FIG. 1 is a picture of a thermally insulated double pipe used in an experimental facility of the applicant of the present application. Regarding a thermally insulated double pipe, reference may be made to patent literature 1 or the like.
According to FIG. 1, an outer pipe 4 which is made of a zinc-galvanized steel pipe, holds a vacuum and has a diameter of 200A (204.7 mm). An inner pipe 2 with a support structure (not illustrated) is provided inside the outer pipe 4. The inner pipe 2 is a stainless pipe and has a diameter of 50A (60.5 mm). A superconducting cable 1 is provided inside the inner pipe 2. Liquid nitrogen is made to flow through a space between the inner pipe 2 and the superconducting cable 1. A space between the outer pipe 4 and the inner pipe 2 is kept airtight in a vacuum under vacuum insulation. In order to cope with heat intrusion through radiation, a multilayer thermal insulation film (MLI) 3 is provided on an outer surface of the inner pipe 2. The multilayer thermal insulation film (MLI) 3 is, for example, formed of a multilayer of plastic films on each of which aluminum is deposited. Even with this structure, there is still observed heat intrusion with an amount of about 1 W to 2 W per unit length (1 m).